OVERPRESSURE AND COMPACTION IN THE LOWER KUTAI BASIN, INDONESIA

ABSTRACT Agus M. Ramdhan The Lower Kutai Basin is a Tertiary sedimentary basin located on the eastern coast of Kalimantan, Indonesia, underlying the area around the Mahakam Delta. Concerning overpressuring, previous workers agreed that the principal mechanism of overpressure generation is disequilibrium compaction, with sand–mudrock pressure discrepancies being present above the transition zone into hard overpressure as a result of lateral reservoir drainage. The pressure data, wireline logs and other data such as temperature and vitrinite reflectance data have been re-examined to analyse the overpressuring in this area. Unloading mechanisms have been considered as alternatives to disequilibrium compaction. The reasons for doing so are the high temperatures in this basin, which promote unloading mechanisms, together with some evidence ignored by previous researchers, from wireline log and vitrinite reflectance data, that also suggest unloading mechanisms play an important role. Clear evidence of unloading has been found in the form of trend reversals in sonic and resistivity logs, without coincident reversals in density logs, and of substantial chemical compaction with mudrock densities exceeding 2.6 g/cm3 at the top of overpressure. In the Peciko Field, a field located in the shelfal area of the basin, mudrock density continues to increase with depth in the overpressured section. All these circumstances are in conflict with the disequilibrium compaction hypothesis; instead, the mudrocks are inferred to be overcompacted. The top of the transition zone into hard overpressure coincides with the onset of gas generation indicating that the gas generation is the principal cause of unloading. Chemical compaction processes must also be ongoing in the overpressured zone, including illitization of mixed layer illite-smectite, illitization of kaolinite, and quartz dissolution and reprecipitation. The result of this research is novel and possibly controversial: there is no other Neogene basin where the role of disequilibrium compaction in overpressure generation has been discounted